Environmental context. Understanding the molecular composition and chemical transformations of organic aerosols during atmospheric aging is a major challenge in atmospheric chemistry. Ultra-high resolution mass spectrometry can provide detailed information on the molecular composition of organic aerosols. Aerosol samples collected in summer and winter at an urban site are characterised and compared in detail with respect to the elemental composition of their components, especially nitrogen- and sulfur-containing compounds, and are discussed with respect to atmospheric formation processes.

Abstract. Organic compounds are major constituents of atmospheric aerosol particles. The understanding of their chemical composition, their properties and reactivity are important for assessing aerosol effects upon both global climate change and human health. The composition of organic aerosols is poorly understood, mainly due to its highly complex chemical composition of several thousand compounds. There is currently no analytical technique available covering a wide enough chemical space to characterise this large number of organic compounds. In recent years ultra-high resolution mass spectrometry has been increasingly used to explore the chemical complexity in organic aerosols from laboratory and ambient samples. In the present study ambient particles <1 µm were collected at an urban site in Cambridge, UK, from August to December 2009. The water-soluble organic fraction of the filters was separated from inorganic ions following a procedure developed for humic-like substance isolation. Ultra-high resolution mass spectrometry analyses were performed in negative and positive polarity. Data in the mass range of m/z 50–350 were analysed for their elemental composition. Summer samples generally contained more components than winter samples. The large number of compounds was subdivided into groups according to their elemental composition. Up to 80 % of the peaks contain nitrogen and sulfur functional groups and only ~20 % of the compounds contain only C, H and O atoms. In summer the fraction of compounds with oxidised nitrogen and sulfur groups increases compared with winter indicating a photo-chemical formation route of these multifunctional compounds. In addition to oxidised nitrogen compounds a large number of amines was identified.